Sn-rich tourmaline from the Land’s End granite, SW England

Kristian Drivenes

Na

Ca

Mg

Fe

Sn

Ti

Al

F

10a

10b

10c

10

10a 10b 10c

1. IntroductionTourmaline and Sn-W mineralizations are closely associated. However, the

commodity of a deposit is not necessarily reflected by the chemical composition

of tourmaline. Sn-rich tourmalines have been observed in Sn-mineralizations

throughout the world, whereas W-content in tourmaline is typically very low,

even in mineralized areas. Tourmaline from a Sn-bearing hydrothermal quartz-

tourmaline rock at Nanjizal, Land’s End, was investigated in order to understand

how Sn is distributed in tourmaline. It is located 10s-100 meters from two small,

formerly worked lodes, and comprises almost exclusively quartz and

tourmaline, with minor K-feldspar, apatite and cassiterite. Tourmaline occurs as

replacements of former granite minerals and as overgrowths on former

tourmaline generations. The relationship with cassiterite is not straightforward,

but typically, cassiterite nucleates on clusters of late-stage tourmaline, and

tourmaline, cassiterite and quartz crystallized simultaneously. The tourmaline

generation coexisting with cassiterite is acicular with strong green/blue-

colorless pleochroism. This association is recognized throughout the Land’s End

granite, but the Sn-content of rocks including this tourmaline generation may

vary from a few 100 ppm to several wt%, the quartz-tourmaline rock at Nanjizal

being an example of the latter. Tourmaline is strongly zoned, both optically and

chemically. Homogenous zones in the tourmaline generation associated with

cassiterite rarely exceed 10 µm, and high-resolution BSE imaging revealed

distinct growth zones less than 50 nm wide.

6. Sn-distribution in tourmalineSn-rich zones are patchy or concentric. The highest

recorded value in a quantitiative analysis was 2.53 wt%

SnO2, and a few zones in the quantmaps show values

around 2.5 wt%. This appears to the the maximum level in

the sample. Several zones with >1 wt% SnO2 were

observed, either as zones surrounding higher Sn-levels, or

as separate zones. The high Sn-levels are typically found in

the center of tourmaline grains forming rims on former

tourmaline generations (e.g. map 10a-c). Submicron

inclusions of cassiterite, Fe- and Ti-oxides are easily

distinguished in the BSE and x-ray maps (Sn, Fe and BSE

in map 7). Also, Sn-rich tourmaline is not necessarily

observed close to cassiterite grains. Thus, the high Sn-

values are not due to cassiterite inclusions, but structurally

bound in tourmaline. High Sn-values (>0.4 wt% SnO2) are

associated with a substantial substitution of Fe for Al

(Figure 2, maps 7 and 10) in the Z-site, and in analyses

with more than ca. 20 wt% FeO, structural formula

calculation indicate that some Fe is present as Fe3+.

However, the Fe-Al substitution is not limited to the Sn-

rich zones (map 10). Trends between Sn and other

elements are erratic, except for a weak positive correlation

with Sr.

The fluid precipitating tourmaline was, at least

periodically, saturated with respect to Sn, but Sn-rich zones

in tourmaline are rare compared to cassiterite. This may

indicate that co-precipitation of tourmaline and cassiterite

was limited, or that Sn-content of the fluid is not the only

parameter controlling the incorporation of Sn in

tourmaline. Cassiterite precipitation from granite-derived

fluids typically requires oxidation, and combined with the

inferred Fe3+ in the Sn-rich zones this may indicate that

oxidation of the ore fluid caused cassiterite precipitation

and the Sn-rich tourmaline zones. Alternatively, the Sn-rich

zones may be the result of local redistribution of Sn from

granite minerals by dissolution-reprecipitation processes.

4. MappingNumerous low-resolution combined EDS/WDS maps ofzoned areas were collected, and areas where Sn wasdetected were selected for high-resolution mapping. Themaps were imported into ImageJ, and the x-ray intensitiesof all elements in spots with known compositions weremeasured. Linear regression of 31 (maps 10a-c) and 20(map 7) spots was used to convert mapped x-rayintensities to mass% (Section 3) This method worked forboth EDS and WDS analyses, since there are no majoroverlaps between the analysed elements. The maps werecollected using a JEOL 8530FPlus at 12 kV, 120 nA, 300 nmstep size, and 500 (map 7) or 750 (maps 10a-c) ms dwelltime, respectively. Maps 10a-c have identical elementscales, showing mass% oxide. B, F, Ca, Ti, and Sn werecollected by WDS, and Na, Mg, Fe, Al, and Si weresimultaneously collected by EDS.

0

10

20

30

40

0 2000 4000

Mas

s% o

xike

X-ray intensity (counts)

Fe (EDS)

0

1

2

3

0 500 1000 1500

Mas

s% o

xid

e

X-ray intensity (counts)

Sn (WDS)Point ID SiO2 TiO2 Al2O3 FeO MgO CaO Na2O SnO2 SrO F Cl Total Si Ti Sn Al Fe3+ Fe2+ Mg Ca Na Name

10a-9 32.01 0.11 20.62 25.89 2.03 0.39 2.38 2.53 0.07 <d.l. <d.l. 86.13 5.85 0.02 0.18 4.44 1.43 2.52 0.55 0.08 0.84 Oxy-Schorl

10b-113 32.24 0.03 18.93 25.61 3.38 0.72 2.19 0.08 0.13 0.19 0.02 83.65 5.96 0.02 0.03 4.13 1.75 2.21 0.93 0.14 0.79 Oxy-Schorl

10c-4 35.74 0.17 30.52 11.22 4.52 0.24 2.14 0.31 0.04 0.19 <d.l. 85.21 6.08 0.02 0.02 6.12 0.00 1.60 1.15 0.04 0.71 Schorl

10c-6 34.52 0.06 20.91 22.98 2.54 0.51 2.35 1.77 0.10 0.08 0.01 85.90 6.24 0.01 0.13 4.46 1.06 2.41 0.69 0.10 0.82 Schorl

7-2 31.46 0.20 16.49 32.04 <d.l. 0.22 2.39 1.27 0.05 0.14 0.01 84.59 6.01 0.03 0.10 3.71 2.07 3.05 0.00 0.05 0.89 Schorl

7-3 31.78 0.09 19.20 29.67 <d.l. 0.38 2.37 0.42 0.04 0.10 0.01 84.27 5.99 0.01 0.03 4.27 1.60 3.08 0.00 0.08 0.87 Oxy-Schorl

7-5 34.26 0.11 31.56 15.34 <d.l. 0.70 1.78 0.02 0.05 0.86 0.03 84.81 6.09 0.01 0.00 6.61 0.00 2.28 0.00 0.13 0.61 F-Schorl

3. Representative EPMA analyses and correlation with mapped intensities

2. Chemical composition of tourmaline

5. Quantitative maps

Map 7

10b 10c 10aNa

Ca

Mg

Fe

Sn

Ti

Al F

BSi

BSE PPL

Figure 2. Classification and discrimination diagrams of tourmaline from the analyzed sample. Most analyses plot in the alkali and hydroxy fields, and classify as schorl and dravite. Sn-rich zones are typically high in Fe and low in Al.

Figure 1. Geological map, field appearance, and examples of optical and chemical zoning of tourmaline in the analysed sample.

Quartz

Cassiterite

Tourmaline

AcknowledgementEPMA analyses were funded by the Norwegian Laboratory

for Mineral and Materials Characterization (MiMaC)

10a-9

10b-113

10c-6

10c-4

7-2

7-3

7-5

400 µm

20 µm

20 µm

20 µm

Corresponding author: Kristian Drivenes, [email protected]